Micromachining aerofoil components

ABSTRACT

The method is suitable for the micromachining of accurately aligned holes in the aerofoil surface of a component such as a jet engine turbine blade or vane which comprises a ground anchorage portion and a cast aerofoil portion. The method uses a laser machining apparatus which is preprogrammed to machine holes in the aerofoil surface when that surface is in a predefined reference position and orientation in the apparatus. Initially the component is mounted in the laser machining apparatus by means of its anchorage portion. At least one probe is then moved over the surface of the cast aerofoil portion to derive the positions of selected points on that aerofoil portion. From the derived position data, angular and linear offset data may be calculated to define the deviation of that cast aerofoil portion from the reference position and orientation defined in the computer memory of the laser machining apparatus. That angular and linear offset data is applied to the computer program resident in the laser machining apparatus to control the movement of the laser head relative to the aerofoil surface and to machine the air holes in the aerofoil surface without having to remove the component from the apparatus.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method and apparatus for the micromachiningof aerofoil surfaces, particularly the turbine blades and vanes of jetengines and other high precision turbines.

2. Description of the Related Art

A turbine blade or vane of a modern jet engine has two distinct parts:an anchorage portion and an aerofoil blade portion. The anchorageportion is ground to create one or more flat planar locating faces whichsecurely and precisely anchor the blade or vane in the engine. The bladeportion is cast, and therefore has a surface which is finished to alower standard of accuracy than the ground faces of the anchorageportion.

Gas turbines generally, and jet engines in particular, run at very highinternal temperatures, often higher than the melting point of the alloyused for the turbine blades or vanes. The blades or vanes are thereforeprotected from melting by creating a film of cooling air over all of theaerofoil surfaces that would otherwise be exposed to the hot combustiongases. The cooling air is discharged from a plenum chamber internally ofthe blade or vane, and out through an array of small and accuratelyaligned and located holes in the aerofoil surface. Those holes aretypically micromachined using a laser mounted with five degrees ofmovement relative to the aerofoil surface of the blade or vane.

A problem associated with the micromachining of the air holes in theaerofoil surface is the need accurately to position the holes relativeto the aerofoil surface itself rather than relative to the mounting. Theaerofoil is typically mounted in the laser cutting machine by itsmachined mounting portion. Although that mounting portion is a highprecision ground sub-element of the component, it may not be preciselyaligned relative to the shaped aerofoil surface. Two alternative methodshave been proposed for ensuring that the laser-drilled holes areaccurately positioned relative to the aerofoil surface rather thanrelative to the mounting portion. Each method requires the use of aseparate piece of setting or measuring equipment in the machineworkshop, and each has its own associated disadvantages.

In a first method, in a setting apparatus the component is initiallypositioned in a compliant mounting which engages with the ground-flatfaces of the anchorage portion of the component. The compliance of thecompliant mounting is such as to provide the aerofoil surface of thecomponent with at least three, and preferably five, degrees of movementin space. The compliant mounting is itself held in the settingapparatus, and jaws and probes of the setting apparatus are moved intoabutment with the aerofoil surface of the component, to grip theaerofoil surface and hold the component by its aerofoil surface in aprecise orientation in space. At that stage the compliant mounting istightened and locked solid, prior to its release from the settingapparatus, so that when the assembly of component and compliant mountingis carried over to the laser drilling machine, the component may be heldby anchoring the compliant mounting against datum surfaces of the laserdrilling machine. Drilling can start immediately, because the aerofoilsurface will be correctly aligned relative to those datum surfaces.Apart from the need for a separate piece of setting apparatus in themachine workshop, for the positioning of the component relative to thecompliant mounting prior to tightening that compliant mounting, thismethod of component alignment suffers from the disadvantage that if thecomponent is mishandled while being transported from the settingapparatus to the laser cutting machine, the component may move in itsmounting so that the accuracy of alignment can be lost.

In a second method for ensuring the accuracy of the laser-drilled holesrelative to the aerofoil surface, the component is first mounted in ameasuring apparatus which engages directly the ground faces of theanchorage portion of the component. Probes or sensors of the measuringapparatus then map the position and shape of the aerofoil surface of thecomponent relative to the ground faces of the anchorage portion, andfrom the output of those probes or sensors a laser drilling controlprogram is compiled, for the control of the laser tool relative to theaerofoil surface in the subsequent micromachining step. The compiledlaser control program and the component are then removed from the testrig and kept together until the micromachining is to take place. Thenthe component is mounted in the laser cutting machine, the controlprogram loaded into the laser cutting machine, and the micromachining ofthe air holes takes place with accurate positioning of those air holesrelative to the aerofoil surface. This method also requires a separatepiece of workshop apparatus, and has the additional disadvantage that ifthe compiled laser control program becomes separated from the componentprior to machining, then the component must be returned to the measuringapparatus for the re-measurement of the aerofoil surface position andthe compilation of a new program. A potentially much greaterdisadvantage of this method is that if two components and their twoaccompanying compiled programs are interchanged accidentally between themeasuring apparatus and the laser machine, then the laser cutting cantake place with the holes being bored in each component using theprogram compiled for the other component. The exchange of programinformation may not be apparent from a visual inspection of the finishedcomponents, but the holes will be offset or misaligned from theiroptimum positions relative to the respective aerofoil surfaces, and thecomponents will be liable to early failure in the extremely demandingconditions experienced in use.

The invention has as its object the avoidance of the above problems andthe establishment of a method of micromachining holes in aerofoilcomponents in which the holes are machined accurately and consistentlyrelative to the aerofoil surfaces and in a manner more economical thanpreviously.

BRIEF SUMMARY OF THE INVENTION

The invention provides a method for micromachining the aerofoil surfacesof a component which comprises a ground anchorage portion and a castaerofoil portion, which method comprises:

mounting the component in a laser machining apparatus by means of itsanchorage portion, the laser machining apparatus being preprogrammed tomachine holes in the aerofoil surface when that surface is in apredefined reference position and orientation in the apparatus;

moving a probe or probes over the surface of the aerofoil surface of thecast aerofoil portion to derive the positions of selected points on thataerofoil portion;

calculating from the derived position data angular and linear offsetdata to define the deviation of that cast aerofoil portion from thereference position and orientation defined in the computer memory of thelaser machining apparatus; and

applying that angular and linear offset data to the computer programresident in the laser machining apparatus to control the movement of thelaser head relative to the aerofoil surface and to machine the air holesin the aerofoil surface.

The method of the invention avoids all potential for misalignment of thelaser head relative to a previously measured or set position of theaerofoil surfaces of the component being machined, because the componentis never removed from one mounting and placed in another during thecourse of the method. The component remains in the laser machine fromthe beginning of the probe test run to establish the position andorientation of the aerofoil surfaces to the end of the machiningprocess. Moreover only a single set of angular and linear offset data iscreated and stored according to the invention, and that data is appliedto the program resident in the laser machine to control, in real time,the movement of the laser head.

Since the laser beam is likely to be a parallel focussed beam ofcircular section, only five pieces of offset data need to be derived:linear offsets along the x, y and z axes and angular offsets about the aand b axes.

The probe or probes may be mounted on the movable platform on which thelaser head is mounted, or on a similar platform, so that essentially thesame motors and/or software can be used for positioning the probe orprobes relative to the component as will subsequently be used foraligning the laser head relative to the component.

Any suitable algorithm may be used for calculating from appropriatederived position data the angular and linear offsets of the aerofoilsurface of the component. Preferably the derived position data areobtained from six probe measurements: four being the positions of twopairs of associated tangential points at which an outer curved edge ofthe aerofoil surface touches tangentially a linear probe; and two beingthe positions of points at which mutually perpendicularly aligned probestouch the aerofoil surfaces of the component.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is illustrated by the drawings of which:

FIG. 1 is a perspective view of a component to be micromachinedaccording to the invention, being a turbine blade; and

FIG. 2 is an end view of the component of FIG. 1, FIGS. 1 and 2 showingthe positions of the six probe measurement points.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates an aerofoil component to be micromachined accordingto the invention. The component illustrated is a turbine vane of a jetengine, and comprises an aerofoil portion 10 and a mounting portion 12.The mounting portion 12 is machined with a series of ground flats 14,16, 18 and 20 by means of which it can be firmly and positively anchoredin the jet engine.

The aerofoil surface 10 of the vane has to have machined therethroughsome tens or hundreds of very accurately located and aligned air holesso that in use a curtain of air is formed over the entire aerofoilsurface, that curtain of air being sufficiently coherent to protect thevane from melting in the high temperatures experienced in a modern jetengine. The method of micromachining those holes according to theinvention is as follows.

First, the entire component is mounted firmly on the platform of a lasercutting machine by clamping directly onto the flat ground surfaces 14,16, 18 and 20. The machine then moves into an initial alignment testmode. A straight probe edge of the machine, illustrated by the thickline 30 of FIGS. 1 and 2, is moved against a leading edge of theaerofoil surface 10 of the vane, until it touches the vane at a pointP₁. The location of the straight probe edge is then recorded in computermemory. The straight edge surface is then turned through an angle α, andagain moved forwardly until it touches the aerofoil surface at a pointP₂ as indicated in the drawings by a thick line 32. The second locationof the straight probe edge is similarly recorded in computer memory. Thestraight probe edge is then moved to another position along the leadingedge of the aerofoil surface 10, and a similar pair of measurements istaken, as shown in FIG. 1 by the thick lines 34 and 36 and the points ofcontact P₃ and P₄. The angle α, and the four measurements taken when thestraight probe edge touched the aerofoil surface at points P₁, P₂, P₃and P₄, are sufficient to define x and y coordinates of the aerofoilsurface 10. The leading edge of the aerofoil surface 10 can beapproximated to a part-cylindrical surface, and the position andorientation in space of the associated cylindrical axis C is thereforeknown.

A point probe, illustrated in FIG. 2 by a heavy line and arrowhead 40,is then moved against a trailing edge of the aerofoil surface 10, apredefined distance D from the axis C. That probe 40 contacts theaerofoil surface 10 at a point P₅, which is sufficient to define theangular rotation of the aerofoil surface 10 around the axis C. Finallythe position of the aerofoil surface 10 longitudinally of the axis C canbe established by turning the probe 40 through 90° and contacting itagainst the end face of the anchorage portion 20, to identify theposition of the point P₆.

From the measured points P₁ to P₆, the precise position of the aerofoilsurface 10 can be accurately established, and according to the inventionthat position is represented by angular and linear offset data whichdefine the deviation of the position of that aerofoil surface 10 from anexpected position as preprogrammed into the laser cutting machine. Theoffsets may be very small, perhaps less than 1° of angular offset orless than 1 mm of linear offset, but irregularities of that order ofmagnitude could seriously interfere with the proper working of theturbine vane if they resulted in the air holes being drilled in thewrong locations.

The above angular and linear offset data is used in real time to offsetthe alignment of the laser beam of the laser cutting machine, so that asthe air holes are drilled into the aerofoil surface 10, they are drilledinto the precise intended locations with reference to the aerofoilsurface 10, and not with reference to the mounting portion 20.

What is claimed is:
 1. A method for micromachining aerofoil surfaces ofa component which comprises a ground anchorage portion and a castaerofoil portion, which method comprises: mounting the component in alaser machining apparatus by means of its anchorage portion, the lasermachining apparatus having a computer memory storing data defining asingle predefined reference position and a single predefined referenceorientation of the aerofoil portion in the apparatus, and data relatingto the position and orientation of holes to be machined in the aerofoilsurface relative to that predefined reference position and orientation;deriving the positions of selected points on that aerofoil portion;calculating, from the derived position data, linear and angular offsetdata to define the deviation of that cast aerofoil portion from thepredefined reference position and orientation; and applying that linearand angular offset data to a computer program resident in the lasermachining apparatus to control the movement of the laser head relativeto the aerofoil surface and accurately to machine air holes in theaerofoil surface even when the component is mounted in the lasermachining apparatus with the aerofoil portion located other than in itsreference position and orientation.
 2. A method according to claim 1,wherein the laser head is mounted on a moveable platform and thepositions of the selected points are derived by using at least one probemounted on the moveable platform on which the laser head is mounted. 3.A method according to claim 1, wherein the derived position data areobtained from six probe measurements: four being the positions of twopairs of associated tangent points of which each pair defines a point atwhich an outer curved edge of the aerofoil surface would touchtangentially two straight lines drawn in space with a known anglebetween them; and two being the positions of points at which mutuallyperpendicular aligned point probes touch the aerofoil portion of thecomponent.
 4. A method according to claim 2, wherein the derivedposition data are obtained from six probe measurements: four being thepositions of two pairs of associated tangent points of which each pairdefines a point at which an outer curved edge of the aerofoil surfacewould touch tangentially two straight lines drawn in space with a knownangle between them; and two being the positions of points at whichmutually perpendicular aligned point probes touch the aerofoil portionof the component.